1. Field of the Invention
The present invention relates to a process for maintaining or improving the mechanical properties of fibers of aromatic copolyamides in alkaline media, i.e. maintaining or increasing the fiber tensile strength, and those shaped articles containing such fibers and hydraulically setting inorganic materials.
Aromatic polyamides (called aramids below) are known as fiber-forming polymers having a good resistance to chemicals. Fibers of aramids which are built up chiefly from para-monomers furthermore are distinguished by good mechanical properties, such as high tensile strengths and elasticity moduli.
The resistance of commercially available fibers of aramids to acid and alkali sometimes leaves something to be desired, so that such fibers cannot be employed without reservation for the production of composite materials which contain strongly acid or alkaline matrix materials. The drop in tensile strength of fibers of such aramids when stored in strongly acid or alkaline media, such as sulfuric acid or sodium hydroxide solution, is explained by partial hydrolysis of the amide bonds of the polymer (cf. H. H. Yang, Aromatic High-Strength Fibers, pages 248-251 and pages 274-276, J. Wiley & Sons). Nevertheless, aramid fibers have already been disclosed which have a resistance to acid and alkali which is improved compared with conventional aramid fibers and a low moisture uptake and the use of which for reinforcing cement has already been proposed (cf. Technische Textilien, Volume 35, October 1992, T128-T132).